For over a hundred years photographic films have been used to capture and display x-rays for diagnostic purposes. In the last ten years or so, digital radiography has become increasingly popular. Digital radiography refers to the application of digital image processing techniques to projection radiography (x-rays). Digitally recorded x-rays are superior to those recorded with photographic film due to the greater dynamic range of the digital recording system. Furthermore, computer image processing techniques provide a wealth of capabilities to study otherwise obscured details within the image.
To take a digital radiograph, a digital radiography imaging unit is positioned behind a subject. A standard radiographic generator directs radiation through the subject to a fluorescent-imaging screen mounted just behind the front surface of the imaging unit. The imaging screen is the conversion media for radiation to visible light. The fluorescent-imaging screen absorbs the radiographic radiation and emits light of a particular wavelength which closely matches the peak sensitivity of a charge coupled device (CCD) camera. A front-surfaced mirror is positioned at a 45 degree angle inside the imaging unit to direct the radiographic image into the CCD camera. The mirror allows the CCD camera to be positioned out of the direct path of the radiation, effectively shielding it from radiation exposure and prolonging its life. A high-efficiency lens reduces the image and directs it onto the surface of the CCD.
The visual image formed by the fluorescent-imaging screen is converted into a digital image by the CCD sensor. A control computer converts the image into a medical image file that can be viewed for clinical diagnosis, enhanced and electronically stored with the patient demographic information in a picture archiving system.
In a digital radiographic image, it is often desirable to reduce the contrast between various parts of the image so that detailed information in all parts of the image can be viewed at the same brightness and contrast (window level and width) settings. As an example, in a chest radiograph, a patient's ribs may be obscured by differing amounts of soft tissue. The result is that some of the ribs may appear to have lower contrast even though they are the same thickness and density as others.